Machine tool monitoring system

ABSTRACT

A system is provided for measuring the time during which a machine tool is performing useful work, such as cutting a work piece. During cutting, the work piece and a work member are in electrical contact with each other. But when both the work piece and work member are rotating with respect to each other and they are not cutting, they are electrically insulated from each other. An electrical circuit having a power source is connected to the machine tool. One terminal of the power source is electrically connected to the work piece and the other terminal is electrically connected to the work member. A first signal is detected during the rotation of the work piece and the work member with respect to each other and the time during which such rotation occurs is measured. A second signal is detected when the machine is performing work and the time during which the machine is performing work is measured.

United States Patent Cother Sept. 5, 1972 541 MACHINE TOOL MONITORINGSYSTEM [72] Inventor: RobertH. Cother, Fullerton, Calif. [73] Assignee:Becton, Dickinson Electronics Com- Primary. Examinier-Alfred E. SmithAttorneyReed C. I awlor [57] ABSTRACT A system is provided for measuringthe time during which a machine tool is performing useful work, such ascutting a work piece. During cutting, the work piece and a work memberare in electrical contact with each other. But when both the work pieceand work member are rotating with respect to each other and they are notcutting, they are electrically insulated from each other. An electricalcircuit having a power source is connected to the machine tool. Oneterminal of the power source is electrically connected to the work pieceand the other terminal is electrically connected to the work member. Afirst signal is detected during the rotation of the work piece and thework member with respect to each other and the time during which suchrotation occurs is measured. A second signal is detected when themachine is performing work and the time during which the machine isperforming work is measured.

6 Claims, 4 Drawing Figures 1 MACHINE TOOL MONITORING SYSTEM BACKGROUNDOF THE INVENTION This invention relates to a new and useful apparatusand method for measuring the time during which a machine tool isperforming work.

Many types of devices have been constructed to measure variousmachine-tool operation times, such as the total time during whichcertain cutting tools are used so that the cutting tools may be replacedat the optimum time. Such apparatus typically employ switches which aremechanically closed at least once during each cutting cycle. Forexample, a cutting head may mechanically close a switch as the cuttinghead reaches its forward cutting position. The closing of such a switchactuates a counting mechanism. Alternatively, some systems detect thecurrent flowing in various branches of a machine control circuit inorder to determine the time which the machine tool spends on each ofvarious machine operations.

More recently, a device has been marketed which is adapted to measurethe cutting time of a machine tool. This device comprises a spindleenergizer. The energizer applies a radio frequency current to thespindle of the machine tool in question, thus producing an alternatingelectromagnetic field in the neighborhood of the cutting tool. A pick-upcoil is positioned near the spindle and work member or cutting tool sothat when the work piece and cutting tool are in contact, a current isinduced by the magnetic field in the pick-up coil, which current isdifferent from the current in the coil when the cutting tool and workpiece are not in contact. This current provides an indication of contactbetween the cutting tool and work piece and thus an indication of whenwork is being performed.

However, in using this latter device, as well as in using some of theother prior art apparatus, care must be taken in positioning thecomponents of the monitoring system. It has been commonly assumed thatthe rotating spindle of a lathe or the like is electrically connected toground. I have discovered, however, that this "assumption is not truefor machine tools which employ a piece of metal work piece) 'or acutting tool work member) which rotate on bearings lubricated with adielectric lubricant, such as oil, grease, or the like. Such a workpiece or work member is electrically insulated from the rest of themachine tool by this lubricant when it is rotating. Taking advantage ofthis discovery, I have invented a novel machine tool monitoring systemfor detecting the time during which such a machine tool is performingwork, such as cutting.

SUMMARY OF THE INVENTION A machine tool monitoring system is providedfor a machine which is adapted to receive an electrically conductivework piece and having an electrically conductive work member. In themachine, the work member and the work piece are electrically connectedtogether when work is being performed on the work piece and the workmember and the work piece are electrically insulated from each otherwhen both work is not being performed on the work piece and when thework piece and work member are moving in a work producing manner withrespect to each other. The monitoring system comprises an electricalcircuit having a first terminal electrically coupled to the work pieceand a second terminal electrically coupled to the work member. Theelectrical circuit also includes electrical signal supply means andmeans for detecting the flow of electrical current between the workpiece and the work member while work is performed on the work piece.

The foregoing and other features of the invention will be understood byreference to the following description taken in connection with theaccompanying drawings wherein:

FIG. 1 is a wiring diagram of a monitoring apparatus embodying theinvention;

FIG. 2 is a block diagram of a second embodiment of the monitoringapparatus embodying the invention;

FIG. 3 is a more detailed partial block diagram of the monitoringapparatus shown in FIG. 2; and

FIG. 4 is a more detailed partial block diagram of the monitoringapparatus shown in FIG. 2.

While reference is made to a lathe throughout the application toillustrate the invention, it will be understood that this invention alsoapplies to various other types of machine tools, such as millingmachines, drill presses, and the like. The invention is described withrespect to a rotating work piece and a stationary, that is, non-rotatingcutter. However it will be understood that the cutting or work membercould rotate, all within the scope of this invention.

In the embodiment of the invention shown in FIG. 1, a machine tool 10 iselectrically connected to ground, such as by mounting it on the floor ofa machine shop with electrically conductive legs.

An electrically conductive work piece 12 is mounted on the lathe 10 bymeans of an electrically conductive chuck 9 and an electricallyconductive live centering member 13. Chuck 9 is mounted on electricallyconductive rotatable spindle 14. Spindle 14 is driven by electric motor19. Centering member 13 is mounted on shaft 16 by lubricated bearings17. Spindle 14 is mounted on lathe 10 by lubricated bearings 15. Spindle14 is sometimes in electrical contact with the bed 11 of the lathe 10,as will be explained more fully hereinafter. Work piece 12 is thussometimes in electrical communication with the lathe bed 11. Anelectrically conductive cutting tool or work member 18 is carried by thelathe.

When work is to be performed, the work member 18 mechanically contactsthe work piece 12 at the same time that the spindle 14 is rotated by themotor 19 of lathe 10, thereby cutting the work piece 12.

In order to measure the time during which the work piece 12 is beingcut, a battery 20 is connected across the spindle 14 and the lathe bed11. A first terminal 22 of the battery 20 is electrically coupled to thespindle 14 while a second terminal 24 of the battery is electricallycoupled to the lathe bed 11. An electrical connector 23, such as a smallbar of copper, is secured to the the terminal 22. This bar of copper 23rides on the periphery of the spindle 14 and provides electricalcommunication between the terminal 22 and the spindle 14. A variableresistor, or rheostat, 26 is connected in series between the battery 20and the first terminal 22. A fixed resistor 28 is connected in parallelacross variable resistor 26 and battery 20. A voltmeter 30 is connectedin parallel with resistor 28 and across variable resistor 26 and battery20.

In certain machine tools, such as lathe 10, the spindle 14 rotates onanti-friction bearings 15 which are lubricated by a dielectriclubricant, such as oil, grease, or the like. A dielectric lubricant is alubricant having a low electrical conductivity. Similarly when thecentering member 13 is in use, it rotates with the work piece, thusrotating the similarly lubricated bearing 17. I have found that adielectric lubricant employed to lubricate anti-friction bearingsprovides electrical insulation below certain voltage limits, between thespindle 14 and centering member 13 and the machine tool bed 11 when thespindle is rotating. Moreover, the electrical insulating properties ofthe film increases, on the average, with the speed of rotation of thespindle 14 and centering member 13. The average breakdown voltage of thefilm thus generally increases with the speed of rotation.

The breakdown voltage of the lubricating film varies even when therotation speed is constant. Such variations occur randomly and are duein part to irregularities in the bearings and in adjacent surfaces.

When breakdown occurs, current flows between the bearings and adjacentsurfaces. When such breakdown occurs, the voltage across the bearing andadjacent surfaces decreases, thus allowing the lubricant to heal itselfor to once again become an insulator. The voltage continues to decreaseuntil the dielectric once again becomes an insulator. This lower voltageis called the healing voltage. Current flow between the bearing andadjacent surfaces is once-again stopped when the lubricant becomes aninsulator and the voltage between the bearing and adjacent surfaces thenquickly increases until the breakdown voltage is reached again.

Repetition of this cycle produces what is called grass, or a noisysignal, between the bearings and adjacent surfaces. This noisy signal isan erratic sawtoothed-wave signal which may be detected across thebearings and adjacent surfaces. The amplitude of the frequencycomponents of this saw-tooth wave signal depends on the speed ofrotation of the bearings.

By way of example, for a 4-inch diameter bearing lubricated with acommercial oil, a grassy signal occurs when a voltage of only about 8 -lmillivolts is applied across the film when the spindle is rotating atabout 4 rpm and when 1 volt is applied across the film when the spindleis rotating at about 1,000 rpm and when 2 volts are applied across thefilm when the spindle is rotating at about 2,000 rpm.

Returning to FIG. 1, when the spindle 14 and centering member 13 arerotating and when cutting tool 18 is not in contact with the work piece12 and when the voltage applied by battery 20 across the spindle 14 andcentering member 13 on the one hand and machine tool bed 11 on the otherhand is below the breakdown voltage of the dielectric lubricant, nocurrent will flow between the spindle l4 and machine tool bed 11. However, when the spindle stops rotating, it sometimes sinks through thefilm of lubricant and comes into electrical contact with the bearingswhich also sink through the lubricating film and come into electricalcontact with the machine tool bed 11. This provides a path forelectrical current to flow between the spindle I4 and the bed 1 1, thusshorting the spindle to the bed.

In the embodiment of the invention shown in FIG. I, the battery 20 is a1 1/2 volt battery and the voltmeter is an 8 millivolt full-scalevoltmeter. When the terminals 22 and 24 are electrically coupled to thespindle 14 and to the machine tool bed 11 respectively, substantially novoltage will be detected by the meter 30 when the spindle 14 is notrotating and cutting tool 18 is not contacting the work piece 12. Whenthe spindle 14 is rotating, such as 10 to 15 rpm, and cutting tool 18 isnot contacting the work piece 12, a substantially higher voltage, suchas 6 to 8 mv, will be detected by the meter 30. When the work is beingperformed on the work piece 12, that is, when the spindle 14 is rotatingat the same time that the work piece 12 and cutting tool 18 are incontact with each other, current will flow between work piece 12,spindle l4 and cutting tool 18 and substantially no voltage will bedetected by the meter 30.

Thus, by reading the meter 30, rotation of the spindle 14 withoutcutting tool 18 contacting the work piece 12 can be distinguished fromthe situations where either the machine is turned off or work is beingperformed. These latter two situations can be distinguished from eachother by observing whether the spindle 14 is rotating. Therefore, byreading the meter 30 and at the same time observing whether the spindle14 is rotating, the time during which useful work, such as cutting, isbeing performed by the lathe 10 may be measured.

In the second embodiment of the invention, shown in FIGS. 2, 3, and 4,an apparatus is provided in accordance with the invention whichautomatically measures the cutting time and the time during which thespindle 14 is rotating. In this second embodiment of the invention, anAC power source or generator is employed. An AC source has the advantageover a DC source of overcoming electrical noise generated by ad jacentmachinery, balancing out any electrolytic processes which may occurbecause of moisture in the lubricant, and avoiding effects ofthermoelectric voltages. The use of AC of a predetermined frequency alsomakes it possible to discriminate against noise from other sources.However, it will be obvious to those skilled in the art that a DC sourcecould also be employed.

In this second embodiment of the invention, an electrical power supplyin the form of an AC generator 50 has a first terminal connected throughcurrent limiting resistor 52 to a first low-impedance brush 60 and agrounded second terminal connected through resistor 54 to a secondlow-impedance brush 62. A second pair of brushes may be employed on thespindle 14 to increase the reliability of the system. The low-impedancebrushes 60 and 62 are composed of a conductive material, such as acomposition of 50 percent to percent graphite with a balance of silver.Brushes 60 and 62 contact continuous slip ring 64 which is composed of aconductive material, such as copper or the like. Slip ring 64 is securedto the spindle 14 of the lathe l0.

Resistor 54 has a resistance which is substantially greater than theresistance of the lathe 10 between the brush 60 and ground when thespindle is not rotating. The bed 1 1 of lathe 10 is electricallygrounded.

FIG. 3 shows the cutting channel of the second embodiment of thisinvention, which corresponds to the lower portion of the block diagramof FIG. 2. The cutting channel is that portion of the measuring devicewhich measures the cutting time of the lathe. FIG. 4

shows the rotation channel of the measuring device which corresponds tothe upper portion of FIG. 2. The rotation channel is that portion of themeasuring device which measures the time during which the spindle 14 isrotating.

Referring to the cutting channel shown in FIGS. 2 and 3, a peakedsaturated amplifier 80 is connected across resistor 54. Amplifier 80includes a narrow band pass filter which is narrowly peaked at 1 KC.Resistor 70 is connected between resistor 54 and one input terminal ofamplifier 80. A second resistor 72 and capacitor 74 are connected inseries between resistor 54 and the second input terminal of amplifier80. A negative feedback resistor 82 is connected between the secondinput terminal and the output of amplifier 80.

The output of peaked amplifier 80 is supplied to halfwave synchronousrectifier 86. Rectifier 86 is also connected to the first terminal of ACgenerator 50.

Amplifier unit 96 is connected to receive the output of rectifier 86.Amplifier unit 96 includes a saturated DC amplifier 100 having an ANDgate 98 at its input. An RC circuit, comprising resistor 88 andcapacitor 90, is connected between the output of the rectifier 86 and afirst input terminal of AND gate 98. An inhibitor 92 is connected at thefirst input leg of the AND gate 98. A DC signal from the rotationchannel (FIG. 4), to be described more fully hereinafter, is suppliedthrough diode 94 and resistor 95 to a second input leg of the AND gate98.

Power amplifier 102 receives the output of saturated DC amplifier 100.The output of power amplifier 102 is supplied to a first input terminalof triac 106 through series resistor 104. A 60 cycle per second powersource 108, such as that supplied by a house wall socket, is connectedto second and third terminals of the triac 106.

A coil 1 is connected between the second terminal of the triac 106 andone terminal of the power source 108. A clock 115, having an inductivelydriven clock motor, is positioned so that the clock motor of the clock115 is driven by the magnetic field generated around coil 110 whencurrent from source 108 passes through the coil 1 10.

The operation of the cutting channel will be explained with reference toFIG. 3. Generator 50 supplies a l kilocycle per second( 1 KC) squarewave AC signal of about 10 volts to the system. This signal isrepresented by curves WCl in the first row of columns I to IV. Theresistances of resistors 52 and 54 are such that a current of about 2 to3 ma flows through the brushes 60 and slip ring 64.

Column I represents the signals at various points in the cutting channelwhen the lathe is idle. When the spindle 14 is not rotating and cuttingtool 18 is not in contact with the work piece 12, current flows directlyfrom the brush 60, through slip ring 64 and spindle 14 (and bearing 17),to the lathe bed 11 which is connected to ground. This, in effect,shorts out the entire cutting channel and only a small signal, if any,will appear across the resistor 54. This is shown by the absence of anysignal, other than the signal WCl of generator 50, in column I.

When the spindle 14 is not rotating but cutting tool 18 is in contactwith the work piece 12, the signal through the cutting channel is againshorted out, this time also through work piece 12, as is shown in ColumnII of FIG. 3.

When the spindle 14 is rotating but cutting tool 18 is not in contactwith the work piece 12, the spindle 14 will be electrically insulatedfrom the lathe bed 11 by a lubricating film as previously described. Inthis situation, as is represented by curve WC2 in column III, a small 1KC signal is developed between the brush 62 and ground. However, due tothe breakdown of the dielectric lubricant between the bearings andadjacent surfaces, as previously described, a noisy secondary signal isimposed on the 1 KC signal. The frequency of this noisy secondary signaldepends upon the speed of the rotation of the spindle 14 and typicallymay be as much as 30 KC to I00 KC. The amplitude of this secondarysignal varies between the breakdown voltage and the healing voltage ofthe lubricating film, as has been previously described.

The noisy 1 KC signal is detected across the resistor 54. Approximately1.5 mv to 4 volts is developed across resistor 54, as is represented bycurve WC2 in column III of FIG. 3.

Peaked amplifier is employed to selectively attenuate the grass or noisyportion of the signal detected across resistor 54 and to selectivelyamplify the 1 KC signal, thereby converting the low voltage noisy signalinto a higher voltage 1 KC signal as is represented by curve WC4 incolumn III. This arrangement thus discriminates against stray noise fromother equipment in the neighborhood.

This 1 kcps signal is rectified by half-wave synchronous rectifier 86which transmits a pulsed DC signal, represented by curve WCS in columnIII. The RC circuit comprising resistor 88 and capacitor 90 converts thepulsed DC signal received from rectifier 86 into a continuous DC signal,represented by curve WC 6 in column III. The time constant of the RCcircuit is long compared with intervals during which the cutter may bemomentarily out of contact with the work piece during intermittent cutsin normal operation. It is also long compared with the period of the ACgenerator. Typically this time constant is about 0.010 seconds.

This DC signal is applied to the inhibitor 92 at one input terminal ofAND gate 98. A DC signal from the rotation channel, to be describedhereinafter, is applied to the other input leg of the AND gate 98through diode 94. The AND gate 98 is actually a high gain switch which,due to the inverter 92, turns off when the voltage at the input to theinverter exceeds 700 mv. This voltage corresponds to 1 mv acrossresistor 54. Accordingly, during rotation when signals appearing acrossthe resistor 54, are less than the l mv, the AND gate is turned on, butwhen larger signals appear across resistor 54, the AND gate is turnedoff. Thus the AND gate 98 transmits signals to amplifier 100 only when asignal of less than 1 mv is applied to inhibitor 92 provided there is apositive voltage signal applied to the other terminal of AND gate 98from the rotation channel.

Therefore, in the present situation where the spindle 14 is rotating butcutting tool 18 is not contacting the work piece 12, a large signal willbe applied to inhibitor 92 and therefore the conditions for passing asignal through the AND gate are not satisfied and no signal istransmitted to the rest of the cutting channel.

When the spindle 14 is rotating and cutting tool 18 is contacting thework piece 12, that is, when cutting is being performed on the workpiece 12, the noisy 1 KC signal is shorted by the contact between thespindle 14 and the work piece 12 and only a very small signal, if any,appears across resistor 54. The signals in the cutting channel for thissituation may be seen in column IV.

At the same time, a rotation signal is applied through diode 94 andresistor 95 to the AND gate 98. This situation satisfies the conditionsfor passing a signal through the AND gate 98 to saturated DC amplifier100. The signal applied to amplifier 100 is a substantially constantamplitude signal, represented by curve WRS in FIG. 4, received from therotation channel. This signal is amplified by saturated DC amplifier 100to produce signal WR7 and by power amplifier 102 to produce signal WR8which is applied, through resistor 104, to a first input of triac 106.

Triac 106 acts as a switch or gate to couple the 60 cps power source 108to the motor of the clock 115 only when a positive signal is receivedfrom power amplifier 102. When such signal is received, that is, whencutting of the work piece 12 occurs, the triac 106 applies an AC signalfrom power source 108 to the coil 1 which in turn operates the motor ofthe clock 1 15.

From the foregoing it will be seen that clock 115 is not operated unlessthere is cutting, that is, the spindle 14 is rotating when cutting tool18 is contacting the work piece 12. The time accumulated by clock 115 istherefore the total cutting time of the lathe 10.

The rotation channel, shown in FIG. 4 and in the upper portion of FIG.2, measures the time during which the spindle 14 of the lathe 10 isrotating. As may be seen in FIGS. 2 and 4, a toothed ring 120 isconnected to, and is adapted to rotate with, the spindle 14. The toothedring 120 is composed of a ferromagnetic material, such as iron. Amagnetic pickup or solenoid, 122, in the form of an iron core composedof a permanent magnet encircled by a coil, is mounted adjacent to thetoothed ring 120. As the toothed ring 120 and the spindle 14 rotate, acurrent is induced in the coil of the magnetic pickup 122. This inducedcurrent provides a signal proportional to the speed of rotation of thespindle 14. A toothed ring 120 having 60 teeth and rotating on spindle14 at approximately 1,100 rpm will induce a signal in the pickup 122having a frequency of approximately 1.1 KC and an amplitude of about -10volts. When the spindle 14 is not rotating, no current is induced in themagnetic pickup 122 and therefore no rotation signal is produced.

The main component of such a rotation signal, when present, is an ACsine wave, represented by curve WRl in FIG. 4. This AC signal isapplied, through a resistor 124 and a capacitor 126, to a first inputleg of saturated amplifier 130. A negative feedback resistor 131 isconnected between the first input leg and the output leg of amplifier130. The amplified AC signal, represented by curve WR2, is applied todemodulator 132 which rectifies the AC signal to produce a pulsed DCsignal, represented by curve WR3. The pulsed DC signal is applied tosaturated DC amplifier 140 after passing through an RC circuit whichcomprises resistor 134 and capacitor 136. This RC circuit produces acontinuous DC signal, represented by curve WR4. The time constant forthis RC circuit is approximately milliseconds when the frequency of thesignal from the pickup 122 is about 1.1 KC.

Saturated DC amplifier amplifies the DC signal and produces a signal ofsubstantially constant amplitude, represented by curve WRS. This DCsignal is applied both to the input of power amplifier 142 and to oneinput terminal of AND gate 98 in the cutting channel, as has beenpreviously described in connection with the cutting channel.

The DC signal, after amplification by amplifier 142, to produce anamplified DC signal represented by curve WR6, is applied throughresistor 144 to a first input of triac 146. Triac 146 is connected topower source 108 and to coil 150. When a signal is applied to the inputof triac 146, the AC signal from the power source 108 is applied to coil150 which induces a current in the motor of the clock 152. Triac 146acts as an open switch unless a signal is applied to it from poweramplifier 142. Therefore, the clock 152 records the total time duringwhich the spindle 14 rotates.

The efficiency, or load factor, of a machine tool may be defined as aratio of cutting time to spindle rotation time. Therefore, by comparingthe time recorded on clocks 115 and 152, the efficiency of operation ofthe lathe 10 may be calculated.

In another embodiment of the invention, not shown, the magnetic pickup122 previously described is replaced by two leads which detect the noisysignal across the brushes 60 and 62. This signal is then applied to arotation channel which is similar to that previously described. But whenthe spindle 14 is not rotating, substantially no signal at all willappear across the brushes.

Thus, a device is provided for measuring the time during which cuttingor similar work is being performed by a lathe or by a similar machinetool. This measuring device does not employ extra insulation, such asinsulating sleeves, and is easily installed on a machine tool. Thedevice of this invention also measures the time during which the lathespindle is rotating so that machine efficiency may be calculated.

It is to be noted that in the foregoing description, the tacitassumption has been made that the spindle is not grounded through themotor. In practice the motor is not usually of the direct drive type, asmight be implied from the figures, but is connected to the spindlethrough gearing or belts and the like. All these drive mechanisms,including the direct drive arrangement, insulate the spindle from theground during rotation of the spindle, and thus do not interfere withthe operation of the system described.

While the invention has been described in connection with a machine toolin which one of the members rotates, it is also applicable to machinetools, such as shapers and planers, in which neither the work piece norcutter rotates, but in which the moving element is supported on alubricant during operation.

It is to be understood, of course, that the invention may be modified inmany ways within the scope of the appended claims. In particular, it isto be understood that the invention is not limited to the specificembodiments disclosed or to the numerical values employed in describingthe invention. Furthermore, many other types of components may beemployed to practice the invention in place of those which have beenspecifically described.

The invention claimed is:

1. A device for measuring the time during which a machine tool isperforming work wherein the machine tool comprises an electricallyconductive work member and is adapted to receive an electricallyconductive work piece and wherein the work member and the work piece areelectrically connected together when both the work member and work piecerotate with respect to each other and mechanically contact each otherand wherein the work piece and the work member are electricallyinsulated from each other when the work piece and the work member areboth out of mechanical contact with each other and are rotating withrespect to each other, the improvement comprising:

means for producing a varying electrical signal in accordance with therotation of said work piece;

first amplifying means for amplifying said rotation signal;

means for converting said rotation signal intova substantially constantamplitude rotation signal;

a first power source connected to said converting means;

first measuring means connected to said power source and adapted to beactuated by said first power source when said substantially constantamplitude rotation signal is transmitted by said converting means;

a second power source having a first terminal coupled to said work pieceand a second terminal connected to said work member;

a resistor connected in series between said second terminal and saidsecond power source, said resistor having a relatively high resistancecompared with the resistance of the work member and said machine tool sothat a substantial no-cutting alternating signal appears across saidresistor only when work is not being performed by said machine tool;

second amplifying means connected across said resistor for amplifyingsaid no-cutting signal;

rectifying means connected to said second amplifying means forconverting said no-cutting signal into a substantially constantamplitude no-cutting signal;

switch means connected to said second amplifier and to said convertingmeans adapted to transmit a cutting signal only when both a rotationsignal is present and when said no-cutting signal is absent from theinput of said switch means; and

second measuring means connected to said first power source, said firstpower source also being connected to said switch means, said secondmeasuring means being adapted to be actuated by said first power sourcewhen a cutting signal is transmitted by said switch means.

2. A device for measuring the time during which a machine tool isperforming work wherein the machine tool comprises an electricallyconductive work member and which is adapted to receive an electricallyconductive work piece and wherein the work member and work piece areelectrically connected together when both the work member and work piecerotate with respect to each other and mechanically contact each otherand wherein the work piece and the work member are electricallyinsulated from each other by a dielectric lubricant when the work pieceand the work member are both out of mechanical contact with each otherand are rotating with respect to each other, the improvement comprising:

means for producing a varying electrical signal in accordance with saidrotation;

first amplifying means connected to said rotation signal producing meansfor amplifying said rotation signal;

means connected to said first amplifying means for converting saidvarying rotation signal into a substantially constant amplitude rotationsignal;

second amplifying means connected to said converting means foramplifying said substantially constant amplitude rotation signal;

a first clock having a clock motor;

a first alternating power source coupled to the motor of said firstclock;

first gate means connected to said second amplifying means and to saidfirst power source, said gate means being adapted to be actuated by saidsubstantially constant amplitude rotation signal to couple said firstpower source to said first clock motor only when said rotation signal istransmitted by said second amplifying means in order to provide ameasurement of the time of said rotation;

first and second brushes coupled to said work piece;

a second alternating power source having a first terminal connected tothe first brush and having a second terminal connected to said workmember and to the second brush;

a resistor connected in series between said second brush and said secondterminal of said second power source, said resistor having a relativelyhigh resistance compared to the resistance of said work member and saidmachine tool so that a substantial no-cutting alternating signal appearsacross said resistor only when work is not being performed by saidmachine tool;

third amplifying means connected across said resistor for amplifyingsaid no-cutting signal;

rectifying means connected to said third amplifying means for convertingsaid no-cutting signal into a substantially constant amplitudeno-cutting signal;

second gate means connected to said third amplifying means and to saidconverting means to receive said substantially constant amplitudeno-cutting signal and said constant amplitude rotation signal, saidsecond gate means having an inhibitor on said no-cutting signal so thata cutting signal is transmitted through the second gate only when arotation signal is present and a no-cutting is absent from the input ofsaid second gate, said second gate thereby transmitting a cutting signalonly when work is being performed by said machine tool;

fourth amplifying means connected to said second gate means;

a second clock having a second clock motor coupled to said first powersource; and

third gate means connected to said fourth amplifying means and to saidfirst power source, said third gate means being adapted to couple thefirst power source to the second clock motor only when said cuttingsignal is transmitted by said fourth amplifying means in order toprovide a measurement of the time during which the machine is performingwork.

3. A device for measuring time as defined in claim 2 further comprising:

a slip ring coupled to said work piece, said first and second brushesbeing adapted to ride on said slip ring, and wherein said first andsecond brushes are low impedance brushes.

4. A device for measuring the time during which a machine tool isperforming work as defined in claim 3 wherein said rotation signalproducing means comprises a toothed ring composed of ferromagneticmaterial adapted to rotate with said work piece and an inductance coiland permanent magnet mounted adjacent to said toothed ring.

5. A device for measuring the time during which a machine tool performswork wherein said machine tool comprises a work piece and a rotatingmember, said measuring device comprising:

means adapted to produce a varying electrical signal in accordance withthe rotation of said rotating member;

first amplifying means connected to said rotation signal producing meansfor amplifying said rotation signal;

means connected to said first amplifying means for converting saidvarying rotation signal to a substantially constant amplitude rotationsignal;

second amplifying means connected to said converting means foramplifying said substantially constant amplitude rotation signal;

a first clock having a clock motor;

a first alternating power source coupled to the motor of said firstclock;

first gate means connected to said second amplifying means and to saidfirst power source, said gate means being adapted to be actuated by saidsubstantially constant amplitude rotation signal to couple said firstpower source to said first clock motor only when said rotation signal istransmitted by said second amplifying means;

a second alternating power source having a first terminal adapted to becoupled to said work piece and having a second terminal adapted to becoupled to said machine tool and to said work piece;

a resistor connected in series between said second terminal and saidsecond work piece, said resistor having a relatively high resistancecompared with the resistance of said work piece and said machine tool;

third amplifying means connected across said resistor for amplifyingno-cutting signals appearing across said resistor;

rectifying means connected to said third amplifying means for convertingsaid no-cutting signal into a substantially constant amplitudeno-cutting signal;

second gate means connected to said third amplifying means and to saidconverting means to receive said substantially constant amplitudeno-cutting signal and said substantially constant amplitude rotationsignal, said second gate means having an inhibitor on said no-cuttingsignal so that a cutting signal is transmitted through the second gateonly when a rotation signal is present and a no-cutting si al is ab entfrom the input of t e gate; fou ampll yrng means connecte to said secondtool is adapted to receive an electrically conductive work piece and hasan electrically conductive work member, the work piece and the workmember being in electrical contact when both the work piece and the workmember are in mechanical contact with each other and are rotating withrespect to each other, the work member and the work piece beingelectrically insulated from each other when both said work piece andsaid work member are out of mechanical contact with each other and arerotating with respect to each other, the steps of:

applying an alternating voltage across the work member and the workpiece; detecting a flow of electrical current between the work memberand the work piece; generating a first signal in accordance with saiddetected current flow; amplifying said first signal; rectifying saidfirst signal to produce a substantially constant amplitude first signal;generating a varying electrical signal in accordance with said rotation;amplifying said rotation signal; rectifying said rotation signal toproduce a substantially constant amplitude rotation signal; combiningsaid first signal and said rotation signal; generating a secondsubstantially constant amplitude signal only when a rotation signal ispresent and when the amplitude of said first signal is below apredetermined magnitude; amplifying said second signal; measuring thetime during which said second signal is present; and measuring the timeduring which said substantially constant amplitude rotation signal ispresent.

1. A device for measuring the time during which a machine tool isperforming work wherein the machine tool comprises an electricallyconductive work member and is adapted to receive an electricallyconductive work piece and wherein the work member and the work piece areelectrically connected together when both the work member and work piecerotate with respect to each other and mechanically contact each otherand wherein the work piece and the work member are electricallyinsulated from each other when the work piece and the work member areboth out of mechanical contact with each other and are rotating withrespect to each other, the improvement comprising: means for producing avarying electrical signal in accordance with the rotation of said workpiece; first amplifying means for amplifying said rotation signal; meansfor converting said rotation signal into a substantially constantamplitude rotation signal; a first power source connected to saidconverting means; first measuring means connected to said power sourceand adapted to be actuated by said first power source when saidsubstantially constant amplitude rotation signal is transmitted by saidconverting means; a second power source having a first terminal coupledto said work piece and a second terminal connected to said work member;a resistor connected in series between said second terminal and saidsecond power source, said resistor having a relatively high resistancecompared with the resistance of the work member and said machine tool sothat a substantial no-cutting alternating signal appears across saidresistor only when work is not being performed by said machine tool;second amplifying means connected across said resistor for amplifyingsaid no-cutting signal; rectifying means connected to said secondamplifying means for converting said no-cutting signal into asubstantially constant amplitude no-cutting signal; switch meansconnected to said second amplifier and to said converting means adaptedto transmit a cutting signal only when both a rotation signal is presentand when said no-cutting signal is absent from the input of said switchmeans; and second measuring means connected to said first power source,said first power source also being connected to said switch means, saidsecond measuring means being adapted to be actuated by said first powersource when a cutting signal is transmitted by said switch means.
 2. Adevice for measuring the time during which a machine tool is performingwork wherein the machine tool comprises an electrically conductive workmember and which is adapted to receive an electrically conductive workpiece and wherein the work member and work piece are electricallyconnected together when both the work member and work piece rotate withrespect to each other and mechanically contact each other and whereinthe work piece and the work member are electrically insulated from eachother by a dielectric lubricant when the work piece and the work memberare both out of mechanical contact with each other and are rotating withrespect to each other, the improvement comprising: means for producing avarying electrical signal in accordance with said rotation; firstamplifying means connected to said rotation signal producing meAns foramplifying said rotation signal; means connected to said firstamplifying means for converting said varying rotation signal into asubstantially constant amplitude rotation signal; second amplifyingmeans connected to said converting means for amplifying saidsubstantially constant amplitude rotation signal; a first clock having aclock motor; a first alternating power source coupled to the motor ofsaid first clock; first gate means connected to said second amplifyingmeans and to said first power source, said gate means being adapted tobe actuated by said substantially constant amplitude rotation signal tocouple said first power source to said first clock motor only when saidrotation signal is transmitted by said second amplifying means in orderto provide a measurement of the time of said rotation; first and secondbrushes coupled to said work piece; a second alternating power sourcehaving a first terminal connected to the first brush and having a secondterminal connected to said work member and to the second brush; aresistor connected in series between said second brush and said secondterminal of said second power source, said resistor having a relativelyhigh resistance compared to the resistance of said work member and saidmachine tool so that a substantial no-cutting alternating signal appearsacross said resistor only when work is not being performed by saidmachine tool; third amplifying means connected across said resistor foramplifying said no-cutting signal; rectifying means connected to saidthird amplifying means for converting said no-cutting signal into asubstantially constant amplitude no-cutting signal; second gate meansconnected to said third amplifying means and to said converting means toreceive said substantially constant amplitude no-cutting signal and saidconstant amplitude rotation signal, said second gate means having aninhibitor on said no-cutting signal so that a cutting signal istransmitted through the second gate only when a rotation signal ispresent and a no-cutting is absent from the input of said second gate,said second gate thereby transmitting a cutting signal only when work isbeing performed by said machine tool; fourth amplifying means connectedto said second gate means; a second clock having a second clock motorcoupled to said first power source; and third gate means connected tosaid fourth amplifying means and to said first power source, said thirdgate means being adapted to couple the first power source to the secondclock motor only when said cutting signal is transmitted by said fourthamplifying means in order to provide a measurement of the time duringwhich the machine is performing work.
 3. A device for measuring time asdefined in claim 2 further comprising: a slip ring coupled to said workpiece, said first and second brushes being adapted to ride on said slipring, and wherein said first and second brushes are low impedancebrushes.
 4. A device for measuring the time during which a machine toolis performing work as defined in claim 3 wherein said rotation signalproducing means comprises a toothed ring composed of ferromagneticmaterial adapted to rotate with said work piece and an inductance coiland permanent magnet mounted adjacent to said toothed ring.
 5. A devicefor measuring the time during which a machine tool performs work whereinsaid machine tool comprises a work piece and a rotating member, saidmeasuring device comprising: means adapted to produce a varyingelectrical signal in accordance with the rotation of said rotatingmember; first amplifying means connected to said rotation signalproducing means for amplifying said rotation signal; means connected tosaid first amplifying means for converting said varying rotation signalto a substantially constant amplitude rotation signal; second amplifyingmeans connected to said converting means for amplifying saidsubstantially constant amplitude rotation signal; a first clock having aclock motor; a first alternating power source coupled to the motor ofsaid first clock; first gate means connected to said second amplifyingmeans and to said first power source, said gate means being adapted tobe actuated by said substantially constant amplitude rotation signal tocouple said first power source to said first clock motor only when saidrotation signal is transmitted by said second amplifying means; a secondalternating power source having a first terminal adapted to be coupledto said work piece and having a second terminal adapted to be coupled tosaid machine tool and to said work piece; a resistor connected in seriesbetween said second terminal and said second work piece, said resistorhaving a relatively high resistance compared with the resistance of saidwork piece and said machine tool; third amplifying means connectedacross said resistor for amplifying no-cutting signals appearing acrosssaid resistor; rectifying means connected to said third amplifying meansfor converting said no-cutting signal into a substantially constantamplitude no-cutting signal; second gate means connected to said thirdamplifying means and to said converting means to receive saidsubstantially constant amplitude no-cutting signal and saidsubstantially constant amplitude rotation signal, said second gate meanshaving an inhibitor on said no-cutting signal so that a cutting signalis transmitted through the second gate only when a rotation signal ispresent and a no-cutting signal is absent from the input of the gate;fourth amplifying means connected to said second gate means; a secondclock having a second clock motor coupled to said first power source;and third gate means connected to said fourth amplifying means and tosaid first power source, said third gate means being adapted to couplethe first power source to the second clock motor only when said cuttingsignal is transmitted by said fourth amplifying means.
 6. In a methodfor measuring the time during which a machine tool is performing workwherein the machine tool is adapted to receive an electricallyconductive work piece and has an electrically conductive work member,the work piece and the work member being in electrical contact when boththe work piece and the work member are in mechanical contact with eachother and are rotating with respect to each other, the work member andthe work piece being electrically insulated from each other when bothsaid work piece and said work member are out of mechanical contact witheach other and are rotating with respect to each other, the steps of:applying an alternating voltage across the work member and the workpiece; detecting a flow of electrical current between the work memberand the work piece; generating a first signal in accordance with saiddetected current flow; amplifying said first signal; rectifying saidfirst signal to produce a substantially constant amplitude first signal;generating a varying electrical signal in accordance with said rotation;amplifying said rotation signal; rectifying said rotation signal toproduce a substantially constant amplitude rotation signal; combiningsaid first signal and said rotation signal; generating a secondsubstantially constant amplitude signal only when a rotation signal ispresent and when the amplitude of said first signal is below apredetermined magnitude; amplifying said second signal; measuring thetime during which said second signal is present; and measuring the timeduring which said substantially constant amplitude rotation signal ispresent.